1. Field of the Invention
The present invention relates to a light emitting device, and more particularly to a light emitting device capable of preventing light, emitted from a light emitting diode or produced is via wavelength conversion, from being lost within the light emitting device.
2. Description of the Related Art
Light emitting devices manufactured with compound semiconductor light emitting diodes can realize various colors and are used for various applications including lamps, electronic display boards, and displays. In particular, since the light emitting device can realize white light, it is used for general lighting and light sources of liquid crystal display panels.
Generally, white light can be obtained by combination of a blue light emitting diode (LED) and phosphors, and one example of light emitting devices that realize white light based on the blue LED and YAG phosphor is disclosed in JP Patent Laid-open No. 2002-064220. However, the technique of this disclosure realizes white light by mixing blue light and yellow light, and exhibits poor properties in view of color reproducibility and color rendering characteristics due to lack of light in the range of red color wavelengths. On the other hand, white light can be realized by three LEDs including a blue LED, a green LED and a red LED, but in this case, the light emitting device exhibits poor color rendering characteristics irrespective of good color reproducibility due to narrow wavelength ranges of light emitted from the LEDs.
In order to solve the aforementioned problems, US Patent Publication No. 2004/0207313 A1 discloses a light emitting device, which comprises a blue LED, a green phosphor and a red phosphor to realize white light, or which comprises a red LED along with the blue LED and the phosphors to realize white light. According to this disclosure, the blue LED is sealed by a light-transmitting resin that contains both green and red phosphors to realize white light having good color reproducibility and color rendering characteristics. Further, the color reproducibility can be improved by adopting the blue LED, green phosphor and red LED. At this point, a light-transmitting resin containing the green phosphor encloses the blue LED and converts a fraction of light emitted from the blue LED into green light. Additionally, there is one example of the light emitting devices that includes a blue LED, a red LED and an ultraviolet LED such that the ultraviolet LED is enclosed by the light-transmitting resin containing the green phosphor to realize white light.
For the light emitting device including the blue LED, the green phosphor and the red phosphor, as disclosed in US Patent Publication No. 2004/0207313 A1, since the green and red phosphors are dispersed in the same light-transmitting resin, green light emitted from the green phosphor tends to be absorbed by the red phosphor. Generally, phosphors exhibit different efficiencies in wavelength conversion according to excited wavelengths. For example, the red phosphor serves to perform wavelength conversion of light emitted from the blue LED into red light, and thus exhibits good efficiency in wavelength conversion from blue light into red light. Accordingly, most of the green light absorbed into the red phosphor is lost through conversion into heat. As a result, in the case where both the green phosphor and the red phosphor are contained in the light-transmitting resin, the light emitting device experiences a lack of green light and a reduction in light emitting efficiency due to a great amount of light lost therein.
Further, light produced via the wavelength conversion in the phosphor can enter the blue LED again. After entering the blue LED, the light passes through the blue LED, and can be absorbed, and thereby lost, into the bottom surface of a substrate where the blue LED is mounted, so that the light emitting efficiency can be further reduced.
For the light emitting device further including the red LED, at least a fraction of light emitted from the red LED can enter the light-transmitting resin containing the phosphor, and other fractions can enter the blue LED or the ultraviolet LED. When red light enters the light-transmitting resin, the light does not excite the phosphor but is instead lost due to diffuse reflection from the phosphor. Further, when red light enters the blue LED or an LED for short wavelength visible light, it can be lost due to reflection within these LEDs. As a result, the intensity of red light is decreased, which necessitates an increase in the number of red LEDs or in drive current of the red LEDs to compensate for the decreased intensity of red light.